Cryogenic engine systems operate by vaporising a cryogenic liquid (e.g. liquid air, nitrogen, oxygen or liquid natural gas, etc.) in an enclosed space and using the resulting pressurised gas to do work by turning a turbine or pushing a piston. It is a known feature of cryogenic engine systems that elevating the peak cycle temperature will increase their work output. In fact, because of the low cycle starting temperature, very high conversion efficiencies of heat into shaft power may be achieved. However, this is at the cost of energy input to produce the cryogenic working fluids required for the cryogenic engine's operation. This energy input is translated into a financial cost which is critical to take into account when evaluating the business case for the use of a cryogenic engine to convert above ambient temperature heat into shaft power.
Very large quantities of waste heat are generated by industrial and transportation processes globally. For example, an internal combustion (IC) automotive engine may only convert 30-40% of the energy available from its fuel input into shaft power; nearly all of the remaining energy is lost as heat through the radiator, intercooler and exhaust systems. Currently, a number of technologies exist targeting primarily high grade waste heat (>>100′C) like turbo compounding, steam cycles, organic rankine cycles and thermo-electric generation. However, very few technologies target the low grade waste heat and yields are typically quite low (e.g. <5% conversion efficiencies).
Cryogenic engines are potentially attractive for waste heat recovery. They use very low temperature working fluids and so can act as a cold sinks for very high yield heat recovery power cycles even with relatively low grade waste heat sources. Examination of the Carnot Efficiency with a liquid nitrogen working fluid and peak cycle temperature of 100′C (373.15K) demonstrates this.
However, unlike many other waste heat recovery devices, the working fluid for these cryogenic engines is typically used in an open cycle (i.e. it is exhausted after use) as a cryogenic liquid production plant is too expensive, inefficient and bulky for small scale static and mobile (e.g. sub-5MW) applications. Consequently, unlike many other waste heat recovery devices, cryogenic engines have substantial operating costs associated with their consumption of working fluid. Additionally, the cryogenic fluid is depleted during the machine's duty cycle.
In general, the prior art has tended to ignore this issue. For example the purpose of the invention disclosed in U.S. Pat. No. 6,891,850 was to use waste heat solely to provide a pressurised stream of gas for some other use, rather than to generate shaft power. Alternatively, attempts have been made to resolve the issue through elevating the cycle temperature to a very high level to raise the specific energy of the cryogenic working fluid. For example, U.S. Pat. No. 4,354,565 discloses a peak cycle temperature of over 900° C. These two approaches have disadvantages when applied to applications that value power generated; the former does not generate any power from the working fluid and the latter is not relevant to the low grade waste heat rejected by IC engines and fuel cells. Additionally, at these higher temperatures closed cycles with other working fluids become feasible and consequently these systems tend to have a high level of complexity.
U.S. Pat. No. 6,202,782 describes a hybrid propulsion system in which thermal storage is used such that a gas turbine may be operated intermittently to power a Rankin cycle. Heat from exhaust gases is stored in an accumulator to drive a secondary expansion cycle.
U.S. Pat. Nos. 4,226,294, 4,359,118 and 4,354,656 disclose a liquid nitrogen or air based power cycle where the primary source of heat is a high temperature furnace. Heat is recovered from a number of other sources (two in U.S. Pat. No. 4,359,118) but there is no provision of a system that can operate dynamically.
US 2010/0083940 uses a cryogenic fluid (liquid air) to cool inlet air for a combustion engine. Although this approach increases the efficiency of the combustion engine, it does not use the cryogen as a working fluid to produce power.
Many instances of cryogenic engine and heat producing power source coupling disclosed in the prior art involve heat being consumed by the cryogenic engine operating as heat is generated. It is sub-optimal to operate a cryogenic engine in this manner. The prior art does not allow anything useful to be done with the heat given off by the heat producing power source's operation when the cryogenic engine is not running.
Therefore, there exists a need for an economically viable means of using a cryogenic engine to convert above ambient temperature heat from any waste source (e.g. an internal combustion engine, fuel cell or other co-located heat generating process) into additional shaft power.
Another aspect of the use of cryogenic working fluids is their low temperature which means that they can provide cooling to co-located processes. However, there exists a need for a system in which cooling, as well as shaft power from a cryogenic engine, is provided by the working fluid of a cryogenic engine. An approach that extracts maximum benefit from every kg of working fluid consumed by utilising both the cold and the work producing capability is likely to maximise the economic benefit of a cryogenic engine used in this manner. In this regard, there also exists a requirement to improve the overall efficiencies of power generation systems in general and systems incorporating cryogenic engines in particular, and whilst some efficiency gains can be achieved through thermal coupling, still further and separate efficiency and indeed economy gains can be made by mechanically coupling a cryogenic engine with another power generation apparatus. Such efficiencies and advantages are achieved by the present invention which allows the overall system to be operated to the best advantage of each of the cryogenic engine and the separate power generation apparatus whilst also allowing each to be of a reduced size relative to the peak power demand. Each engine can be optimized for power production within the desired band of performance and can be both smaller and more efficient than might otherwise be possible if it alone was to be providing the total power output. Demand is met by intermittent operation of one or other or both engines depending on the power demand and this is in stark contrast with the prior art which tends to use the cryogenic engine at full output all the time.